Turning a Nested 2D Loop into 1D

In GPU programming, you might sample a 2D window with a nested. However, a nested loop might be more inefficient than 1 loop.

This post is an example of how to sample a 3x3 window of offsets in 1 loop.

Source Code

// Get required data to calculate main window data
const int WindowSize = 3;
const int WindowHalf = trunc(WindowSize / 2);

// Start from the negative so we can process a window in 1 loop
[loop] for (int i = 0; i < (WindowSize * WindowSize); i++)
{
   float2 XY = -WindowHalf + float2(i % WindowSize, trunc(i / WindowSize));
}

i#

X

Y

0

-1 + (0 % 3)

-1

-1 + trunc(0 / 3)

-1

1

-1 + (1 % 3)

0

-1 + trunc(1 / 3)

-1

2

-1 + (2 % 3)

1

-1 + trunc(2 / 3)

-1

3

-1 + (3 % 3)

-1

-1 + trunc(3 / 3)

0

4

-1 + (4 % 3)

0

-1 + trunc(4 / 3)

0

5

-1 + (5 % 3)

1

-1 + trunc(5 / 3)

0

6

-1 + (6 % 3)

-1

-1 + trunc(6 / 3)

1

7

-1 + (7 % 3)

0

-1 + trunc(7 / 3)

1

8

-1 + (8 % 3)

1

-1 + trunc(8 / 3)

1